A coextruded polyethylene blown film is particularly suited to the manufacture of packaging, it being possible to form such packaging by thermal welding due to the first weldable outer layer of the polyethylene coextruded film, in which case either a film blank or a continuous web is formed into packaging by folding or at least two film blanks or two film webs are connected by hot-seal welds.
The coextruded polyethylene blown film can be used both in a so-called FFS method (Form Fill and Seal) where bag-like packaging is formed immediately during packing of the product, and in the manufacture of prefabricated packaging that is subsequently filled.
Moreover, the coextruded polyethylene blown film can also be used as a cover film in a tray pack, or as a label or the like.
The foaming of a film layer can be done for various reasons. First, foaming results in a lower mass per unit area relative to the resulting thickness of the film and thus saves material. Particularly in combination with other unfoamed film layers, it usually also results in an improvement of the mechanical characteristics relative to the quantity of plastic used as a result of the greater thickness. For example, if a foamed core layer is combined with unfoamed covering layers according to the preamble of claim 1, it results in a kind of plywood effect in which the further separated outer layers can be deformed less readily relative to each other as a result in the increase in volume of the core layer compared to an unfoamed design. What is more, the other physical characteristics of the resulting coextruded film are also influenced by the foaming of at least one film layer.
According to CA 1,145,724, EP 0 512,740, JP 2004-91024, EP 1,761,437, JP 2007-230637, DE 10 2011 051 193, a foamed film layer is used as a mechanical buffer in order to increase the puncture resistance or compensate for mechanical deformations from the filled product to a certain extent.
At least one foamed film layer can also be used for thermal insulation. Such approaches are known from JP 2001-130586, U.S. Pat. No. 6,913,389, KR 2004-0005806, and KR 2004-0007381.
Moreover, a foamed film layer can also result in weakening in a film that promotes tearing in a desired manner. As a result of the lower density and the free spaces within the at least one foamed film layer, it can be torn relatively easily in its direction of thickness. Depending on the specific design, foaming can also facilitate the separation of layers relative to an adjacent film layer, it being possible to exploit these characteristics for the manufacture of tear-open packages. Furthermore, if the film is torn perpendicular to its thickness, particularly in the event of a tear propagation along the manufacturing direction, weakening occurs as a result of the foaming. Depending on the manufacturing process, pronounced anisotropy can also be attributed to the fact that the pores or cells formed during foaming are aligned in a manufacturing direction, making tearing particularly easy along the longitudinal direction of these cells or pores (i.e. along the manufacturing direction). The use of foamed film layers to produce defined tearing characteristics is described, for example, in GB 2,110 215 B, U.S. Pat. Nos. 4,762,230, 4,781,294, EP 673,756, JP 3823967 and DE 20 2005 002 615.
The foaming of at least one film layer also leads to increased roughness or undulation of the film surface, this effect being utilized advantageously in DE 2,038,557, DE 37 22 139 C2, DE 196 53 608 B4, JP 2001-055242, EP 1,237,751. However, undulation or roughness resulting from foaming is also undesirable in many cases, for example if a surface of a package is to have an appearance that is as uniform, smooth and high quality as possible.
Another effect of a foamed film layer that is known from practice is that of the film having a higher level of cloudiness and opacity. For example, through foaming, the use of colored particles can also be reduced in order to produce opaque or merely translucent film. The increasing of the opacity and exploitation of this effect are described in EP 83,167.
Various methods are known for manufacturing films with at least one foamed film layer. The foaming can particularly be done by a chemical reaction or a physical process. For example, substances contained in the polymer melt during extrusion can vaporize or react and form a gas. In this context, it is also possible to introduce microspheres into the polymer melt having a propellant within a meltable coating.
During physical foaming, a propellant is added to the molten plastic mass in the extruder under high pressure. Examples of suitable propellants are water, nitrogen or carbon dioxide.
Especially uniform, good mechanical characteristics are achieved if the foamed layer has an especially fine-celled foam structure that can be formed using the so-called MuCell method, for example. Apparatuses for executing the method or for retrofitting standard extruders are sold by Drexel Inc., USA. The MuCell method is described particularly in U.S. Pat. Nos. 5,866,053, 6,051,174, EP 923,443, EP 1,275,485, EP 377,650, EP 580 777, U.S. Pat. No. 6,231,942, EP 996,536, EP 1,040 158, EP 1,131,387, EP 1,283,767, EP 1,337,387, EP 1,539,868, EP 1,337,387, and EP 1,575,763. The present invention relates particularly to polyethylene coextruded films in which the foamed core layer is formed according to the described MuCell method.
During extrusion, a propellant is added to the melt for the core layer to be foamed that brings about foaming during extrusion or immediately after emerging from the extrusion gap. On exiting the coextrusion gap, the propellant added previously to the melt under pressure expands suddenly. The propellant is usually present within the extruder as a supercritical fluid that combines the incompressibility of a liquid and the dissolution characteristics of a gas. The propellant goes into solution in the polymer melt and forms a single-phase system distributed in the plastic melt. As a result of a quick drop in pressure upon emerging from the extrusion die, nucleating particles form in the polymer melt. The gas is released from the melt, and a very fine, uniform foam structure is formed. The particles in the core layer can promote the formation of an especially large quantity of especially small nucleating particles. The particles thus do not serve in the framework of the invention as a favorable volume material, or at least not exclusively; rather, they are used as a functional component for improving the film characteristics, namely for the formation of an especially large quantity of especially small pores or cells. However, the particles can also be referred to as nucleating agents.
To enable formation of cells or pores that are as uniform and fine as possible, it has proven advantageous to maintain the solubility pressure in the melt at a high level for as long as possible in order to then achieve a sudden drop in pressure only as the melt emerges from an extrusion die. While extrusion dies for blown film extrusion or wide-slot extrusion often expand like a cone or are straight, when foaming is performed in the framework of a MuCell method, narrowing the gap right at the end can be advantageous in order to maintain the pressure of the melt high for as long as possible, thus achieving a drop in pressure that is as steep as possible. If the gap is too narrow, however, excessive resistance occurs during extrusion, and irregularities in the structure can occur even in the case of multilayered coextrusion with a foamed core layer and unfoamed outer layers with partial breakaway of the melt flow. Such technical solutions are known from US 2012/0228793, WO 2013/148841.
Other methods for manufacturing a film or a plastic body with at least one foamed layer are also known from U.S. Pat. Nos. 4,473,665, 4,522,675, EP 580 777, EP 843,246, TW 384271, U.S. Pat. No. 6,403,663, EP 1,189,978, U.S. Pat. No. 7,341,683, EP 1,857,501, and EP 1,888,676.
Various extrusion apparatuses are the subject matter of EP 1,075,921 and EP 1,719,600.
Printed publications U.S. Pat. Nos. 4,533,578, 4,657,811, EP 237,977, U.S. Pat. No. 5,000,992, EP 5,553,522, JP 11079192, U.S. Pat. No. 6,096,793, EP 1,088,022, JP 2002-154555, EP 1,297,067, EP 1,646,677, JP 2006-027185, JP 2007-045047, JP 2007-045046, EP 1,857,501, EP 1,973,733, EP 2,043,857, WO 2008/100501, WO 2009/155326, EP 2,258,545, JP 2013-111811, EP 2,668,036, KR 2013-0100597, WO 2013/179947 relate to other films or plastic objects with a foamed layer or the manufacture thereof.
As explained above, the present invention specifically relates to a coextruded polyethylene blown film, particularly for packaging, with a thickness between 20 μm and 250 μm comprising a foamed, filler-containing core layer between a first unfoamed weldable outer layer and a second unfoamed outer layer. The coextruded polyethylene blown film can particularly be manufactured using the previously described MuCell method, with blown film coextrusion being preferred.
In known polyethylene coextruded films with the features described above, the foamed core layer results in the drawback that the outer layers are uneven due to the underlying core layer with bubble-like cells and pores, and the visual appearance is impaired.